This invention relates generally to a method and apparatus for carrying out the transfer of reactants such as oxygen and/or the transfer and separation of products from reactions, which can be biological reactions or the like. Biological reactions using immobilized microorganisms have been the focus of much recent research and development. Notable commercial processes using immobilized cells include glucose isomerization, raffinose hydrolysis and amino acid production. Other useful reactions such as antibiotic modification and organic acid formation and degradation have been extensively studied and some are near the stage of commercial application.
The widespread interest in immobilized cell systems is well justified given the advantages of these systems over freely suspended cells. The most obvious advantage is the continuous use of biomass which is retained in the reactor. The yield with respect to product is thus increased due to the decrease in the amount of biomass synthesis. Cell immobilization also provides the means to make batch processes continuous, and it can be employed with resting cells for continuous secondary metabolite production. The high cell densities achieved by immobilization yield faster reaction rates. Finally, by removing or reducing the cells suspended in the medium, immobilization can improve the rheological properties of the medium while increasing the effective densities of the microorganisms. For these reasons, many-fold productivity increases have been realized with immobilized cell reactors.
There are, to be sure, several problems associated with immobilized cells systems which are due, in general, to the biological system, the immobilization technique, or the reactor system. Immobilization may result in a loss of some of the desirable catalytic activity either because of enzyme inactivation during immobilization, or because of diffusional barriers that decrease substrate access to or product removal from the cells. Packed beds as immobilized cell reactors have the disadvantages of being mass-transfer limited, being subject to plugging, and using only a small fraction of the available cells for biocatalysis. Hollow fiber cell reactors, such as those disclosed in U.S. Pat. No. 4,201,845 to Feder et al, have mass transfer limitations through the membrane that separates the nutrients from the cells.